1. Field of the Invention
The present invention relates to a communication protocol, and more particularly to a MAC (Medium Access Control) protocol in a LAN (Local Area Network) communication system.
2. Description of the Related Art
Wireless LAN (Local Area Network) services conducted through a public network, which can be implemented by a next-generation wired/wireless communication integration technique, will become an important requirement in next-generation communication markets. A wireless LAN is expected to qualitatively change the IT (Information Technology) industry. In particular, because the wireless LAN coupled to the public network is associated with mobile communication terminals such as a notebook computer, a PDA (Personal Digital Assistant), etc., the quantitative growth of related industries is expected to be accelerated. The number of users requiring multimedia data continuously increases in current communication networks. Similarly, the users will also desire to receive the multimedia data through the wireless LAN.
The wireless LAN being widely used has been implemented on the basis of an IEEE (Institute of Electronic and Electrical Engineers) 802.11 standard. A basic access in a MAC (Medium Access Control) hierarchy protocol of the IEEE 802.11 standard is performed by a DCF (Distributed Coordination Function). The DCF is based on CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance). Through the CSMA/CA, all stations can equally occupy a common medium. The DCF in the MAC hierarchy protocol makes the stations wait for a period of time equal to a DIFS (Distributed Inter Frame Space) using a random back-off algorithm so that the medium can be equally assigned to the stations using one medium. The CSMA/CA also includes an optional PCF (Point Coordination Function), which is used to set up an access point as a point coordinator. In this function, the point coordinator assigns a priority and processes predetermined time-bounded information. The PCF makes the stations wait for a period of time equal to a PIFS (Point coordination IFS (Inter Frame Space)), and shorter than the DIFS, using the random back-off algorithm. Data used in the PCF has a higher-order priority than that used in the DCF. However, the PCF cannot set a plurality of priorities, but can only identify whether there is a priority. Further, the PCF allows the station to access the medium irrespective of the priority in a back-off time.
The DCF for the basic access in the MAC hierarchy protocol makes all the stations equally occupy the medium in relation to all data. Accordingly, delay-sensitive data such as video, voice and audio data, and asynchronous data such as e-mail data and file data for transmission can be equally processed. When a station transmits an MPDU (MAC Protocol Data Unit) through a conventional DCF or PCF access method, it waits for the period of time equal to the DIFS or the PIFS and then generates the back-off time using a pseudo random number, thereby performing a back-off procedure. The stations pseudo-randomly generate the back-off time irrespective of the priority in the conventional DCF or PCF access method, thereby maximize the equal use of occupying the medium. Further, the access method is conventionally performed on the basis of the priority. In other words, when the MPDU having the priority is transmitted, the PCF based on the priority allows the station to quickly occupy the medium by using the PIFS shorter than the DIFS used in the DCF, thereby reducing a period of waiting time. However, the PCF cannot assign different priorities through several stages. In the PCF, the back-off time is equally set irrespective of the priority and the back-off procedure based on the equally set back-off time is performed.
FIG. 1 is a view explaining a conventional basic access method.
Referring to FIG. 1, stations occupy a medium through an access contention. When a station attempts a data transmission in the case where the medium is in a busy state, the station trying to perform the data transmission recognizes the busy state of the medium. Thereafter, the station waits until a current transmission stops. After the current data transmission stops, the station waits for a period of time equal to an inter-frame space, e.g., IFS, PIFS or DIFS, and sets a back-off time on the basis of a value of a generated CW (Contention Window). Thus, the station waits for an MPDU transmission while performing a back-off procedure.
FIG. 2 is a view explaining a method for occupying a medium in stations on the basis of the conventional basic access method. In connection with FIG. 2, a detailed description will be given of a method for occupying the medium in the stations under an access contention through a back-off procedure based on a back-off time. Referring to FIG. 2, the stations occupy the medium irrespective of a priority and transmit a frame of an MPDU. First, a station sets a waiting time based on a value of the CW pseudo-randomly generated and occupies the medium after the waiting time based on the value of the CW. After the back-off time is set, a back-off slot corresponding to a period of idle time is removed. Accordingly, as the station has a longer waiting time, an opportunity to transmit the MPDU frame is increased. Through this procedure, the stations can maximize the equal use of the medium through the access contention. Any station having the MPDU frame can set the back-off time by generating a pseudo random number. The stations have the same value of the CW.
FIG. 3 is a flow chart explaining a conventional method for occupying a medium through a back-off procedure. FIG. 3 is a flow chart illustrating a control method to be performed in a MAC hierarchy module when a station accesses the medium in a network in which a plurality of stations can be coupled to one medium.
Referring to FIG. 3, the station tries to transmit a frame of an MPDU at step 20. If the station fails to transmit the MPDU frame, it delays the transmission of the MPDU frame for a period of time equal to a DIFS or PIFS at step 22. A pseudo random number for each of all the stations is generated within a constant range irrespective of a priority at step 24 and a value of a CW is generated at step 26. A back-off time is generated on the basis of the pseudo random number and the CW value at step 28. The station occupies the medium using the back-off procedure and tries to re-transmit the MPDU frame at step 30.
After the station performs the back-off procedure, it is determined at step 32 whether there is a collision between data transmissions. If there is the collision between data transmissions, the station determines at step 34 whether the number of re-transmission times is less than a reference value representing a limit of the number of re-transmission times. If the number of re-transmission times is less than the reference value, the CW value is changed to a next CW value greater than the CW value according to the equation 2k−1, where k is a positive integer, at step 40. The station performs the back-off procedure and tries to re-transmit the MPDU frame by occupying the medium. When there is the collision between data transmissions, the MPDU frame is re-transmitted the limited number of re-transmission times. If the MPDU frame has been successively transmitted or the MPDU frame has been re-transmitted the limited number of re-transmission times, the CW value is re-set at step 36. Through this procedure, medium occupation is performed so that the station can transmit the MPDU frame. The medium occupation is performed under same conditions irrespective of contents of the MPDU frame. Thus, the stations can equally occupy the medium. Because the conventional method shown in FIG. 3 does not process time-bounded information, there is a problem in that it is difficult for data to be processed within a required time period.
When video data, audio data and voice data being time-bounded information are processed in the DCF, data transmission is delayed to cause the communication of the video data, the audio data and the voice data to be cut off in the middle of communication or to cause the video, audio and voice data to be lost. At this time, the communication of the video data, the audio data and the voice data cannot be properly performed. In other words, the MAC hierarchy protocol supports general data communication on the basis of the wireless LAN (IEEE 802.11) widely used, but cannot properly process the time-bounded information such as the video data, the audio data and the voice data. An improvement of the conventional method is seriously needed to effectively process the video data, the audio data and the voice data and perform data communication without loss of data. An optional PCF provided by the IEEE 802.11 standard is used for real-time application. However, the optional PCF cannot assign different priorities through several stages and can only recognize whether there is a priority or not. Further, the optional PCF is used to set up an access point as a point coordinator. There are two operating modes including an infrastructure mode and an ad-hoc mode in the IEEE 802.11 standard. The optional PCF is operable in the infrastructure mode. However, the optional PCF is not operable in the ad-hoc mode.
In other words, the MAC hierarchy protocol appropriately supports the general data communication on the basis of the wireless LAN (IEEE 802.11) widely used, but cannot process the time-bounded information such as the video data, the audio data and the voice data. An improvement of the conventional method is seriously needed to effectively process the video data, the audio data and the voice data and perform data communication without loss of data.